Ingnoring distribution problems, how many gallons of water do you need to “pour” onto an acre of land to equal one inch of rain fall? Feel free to answer in metrics. 
1 acre = 43560 sq ft. * 1/12 ft = 3630 cu. ft.
Using the google calculator: 3630 (cu feet) = 27 154.2857 US gallons
So about 28,000 gallons per acre for an inch of “rain.” Thanks.
Ok, next dumb question: how many inches of rain does a region need for land to be arable (assuming soil quality is ok)? I see the average worldwide rainfall is 39 inches/year. Over what range of rainfall would you need to grow a vareity of staple crops (garden or local market crops) or at least some crops that are on the less “thirsty” end of the scale? Or does soil quality make a huge difference in this calculation?
There’s a huge range of variation here. There are some crops that will provide a modest yield in dry climates. And you can grow just about anything in the desert if you water it enough. Most farms in the US use irrigation (some much more than others). In many cases, the irrigation isn’t necessary but it helps improve crop yields.
For one upper bound for “enough” water for current agricultural practices, I know that farmers in northern Ohio get enough rainfall that most don’t bother to irrigate. There, the annual rainfall is approximately 40 inches/year.
Abuse of the equals sign, B-
The Google calculator can do it in one step:
http://www.google.com/search?q=1/12+acre-feet+in+gallons
In case the URL gets truncated, do a Google search on “1/12 acre-feet in gallons”.
You also have to factor in when the rain falls.
In the Pacific Northwest, we get some rain year-round, but the bulk of it comes during the winters. Not much help during a summer growing season unless you have the technology to store the water and use irrigation. (In fact, we’re pretty reliant on melting snow in the Cascades for our summer water. Low winter snowfall means droughts in the summer.)
Parts of Arizona actually get significant amounts of rainfall, but it comes during their “monsoon season” as short, heavy deluges. It can’t all soak in and runs off with impressive flash floods, etc. So this would also not be very helpful for crops.
Or even “1 acre inch in gallons”
How so?
ETA: Nevermind, I was reading the first period as a sentence break, whereas it’s probably just part of the abbreviation “ft.”, in which case, I agree, abuse of the equals sign
What? Communicate clearly, D-. :smack:
What is abusive about the equals sign there? One acre is indeed 43,560 sq. ft.
but one acre does not equal 43,560 sq. ft. times 1/12 ft.
The demarcation between arable land and grazing land is known as
Goyder’s_Line, which broadly follows the 10" isohyet in Australia.
That is a winter rainfall pattern for winter cereals.
The question is of reliability and distribution, not amount.
Well, then that’s just incorrect operator precedence, then, not abuse of the equals sign specifically. You all get an F.
Well, I guess you could say “(1 acre = 43560 sq. ft.) * 1/12 ft = 3630 cu. ft.” reads acceptably, and therefore, since parenthesization makes things acceptable, it’s just a precedence issue, but it’s not really as though = is an operation returning a value which we then multiply by 1/12 ft to get 3630 cu. ft. The parenthesized version is still using some notational “abuse”; the only reason it reads a little better is because that particular kind of notational abuse is more standardly employed. At any rate, I’m not inclined to call it a precedence issue. But, whatever. I’m going to stop this nitpicking hijack before it gets out of hand.
Not all of the rainfall will go into uptake by the plants. The amount that runs off rather than soaking in is partially a factor of the type of soil the plants are growing in, the slope of the land, and any evaporation that is taking place.
So this is when you switch from Uncle Toby’s to Weetbix?
I considered splitting all of my operations to separate lines to avoid abusing the equal sign, but given the simple nature of the calculation, I thought it would be obvious what happened. I agree that it’s a bit unclear, but come on, this isn’t a math exam here!
Right, like I said, I have no desire to badger on with the nitpicking hijack. (The only reason I entered it in the first place was because I originally didn’t see what naita was talking about and was thus curious.)
Hey - I recently did a calculation like this. I wanted to know how many gallons of rain fell on our little 4.5 sq-mile town when we got a recent 8 inch drenching. (Answer - about 11 million gallons). It was important because we’re having discussions about the capacity of our storm and sewage drain systems.